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Dolly the sheep has DNA damage that might cause premature ageing, according to research by scientists who created the clone in 1996.

The team from the Scottish biotechnology firm PPL Therapeutics has found Dolly and two other cloned sheep have shorter than normal telomeres - sections of DNA at the end of chromosomes thought to be involved in ageing.

Telomeres have been likened to the protective caps on the ends of shoelaces that prevent the lace from unravelling over time.

Only in living cells, it's thought the telomeres on the ends of chromosomes become progressively shorter with multiple cell divisions, until they reach a point when the chromosomes become unstable and prone to damage. This may be a mechanism to ensure the death of cells once they reach a certain age.

In today's Nature, the team report that Dolly, who was cloned from a cell taken from a six-year-old sheep, had telomeres about 20 per cent shorter than other sheep of a similar age conceived naturally.

The finding is not unexpected because cells used for cloning have to be cultured - allowed to divide many times in laboratory dishes - a process that is known to shorten telomeres. And in Dolly's case, the shortening was compounded because she was derived from a cell from a middle-aged animal.

But the confirmation is of interest because it should allow scientists to test theories linking shortened telomeres with premature ageing.

PPL's research director, Alan Coleman, said the team had not observed any signs of premature ageing in Dolly yet, but he conceded they might show up later.

He says it is impossible to predict precisely how long the world famous sheep will live. It is not known what length of telomere is critical for life.

Previous studies by researchers in the United States have shown mice bred to have shorter telomeres, did indeed show premature ageing - their hair turned grey, they developed cancers and died earlier than usual.

But other studies on mice cloned from cells of previous clones over multiple generations showed no adverse effects.

If the telomere theory is proven correct, it may place limits on the potential of exciting new medical technology arising from developments in cloning.

Hopes of taking master cells from cloned human embryos and growing them into perfectly matched organs for transplant surgery could be thwarted if the process of creating the organs left them with a short life.

Just as second-hand car parts may be too "clapped out" to revive a car, so these organs may end up being prematurely aged by the cell-culturing process used in their creation. But limiting cell culture times might overcome the problem.